I can't open IDE in other desktop.

you can't run several WebStorm instances, as they can't access their caches concurrently... So, any attempt to run a new instance results in shutting the process down.
If you really need running 2 instances at the same time, you can install WebStorm to a different folder and specify a different configuration and caches directories for it (https://youtrack.jetbrains.com/issue/IDEABKL-5934#comment=27-182509).
Follow the instructions at: https://intellij-support.jetbrains.com/entries/23348963-Changing-IDE-default-directories-used-for-config-plugins-and-caches-storage

Main steps:
1. Install WebStorm to a different location.
2. Change idea.properties file for this installation. Remove comments for these properties:
idea.config.path
idea.system.path
idea.plugins.path
idea.log.path

After that you will be able to run two instances of WebStorm at the same time.

Windows 10 has this nifty feature of multiple desktops you can switch between.

When IntelliJ (in my case PhpStorm) is already opened, a second click to the shortcut in taskbar/desktop won't have a effect.

What works is: Return to Desktop 1, open your project in a separate window, move it to Desktop 2. But this is pretty much anoying.

Couldn't you check if there is a running instance when starting a second one and ask for what to do?

Something like: Another instance of IntelliJ is currently running. What do you want to do?

- Focus running instance
- Open new Window (on the desktop I'm currently working in) 

I'm posting this a little late, but in the hope that it will help someone with a similar question. I use PHP Storm and wanted to do this as well -- different desktops for different client projects -- and found I could get it to work by opening the IDE on my first desktop, opening the additional project in a new PHP Storm window (the program gives the option when choosing File...Open Recent Project), and once the new IDE window is open, invoking the enhanced task manager (Windows + Tab) and dragging the new IDE window onto the second desktop.

I need to work with multiple projects and I use multiple Windows 10 desktops to organize my work. Opening new WebStorm instances for this workflow is annoyingly frustrating, since I too have discovered that the only working option is to find an already open instance, and then open the project there and then drag the window to the desired desktop.

It is typical for me to have at least five desktops in use at the same time, and I often have to go "fishing" for the already open Webstorm in order to open a new instance.

It might sound like a small thing, but it is eating away from the experience quite a bit when the very first thing I do with the IDE (opening it) already causes frustration and annoyance. I would love to see this fixed, so that by clicking the icon, using the Toolbox launcher or starting from command line would all result in opening a new IDE in the currently active desktop. Right now, if you are on the "wrong" desktop clicking the icon or launching from Toolbox does nothing. That can't be right.

There's workaround for that. It throws error, but it works :) 

Edit your IDE shortcut 

1) Make shortcut of IDE
2) Add `--new-window` to the end of shortcut properties / command
  - So it becomes something like "C:\Program Files\WebStorm\bin\WebStorm.exe" --new-window (also works on Linux the same way WebStorm --new-window)

That will open up new window and show error on corner of screen "Cannot create C:\Program Files\JetBrains\WebStorm new document --new-window must be saved to a different location.", that error can be hidden or just ignored

3) Open whatever file you wanted